1. Field of the Invention
This invention relates to coated iron oxide pigments particularly iron oxide magnetic pigments and the utilization of such coated iron oxide magnetic pigments in magnetic tape. More particularly, this invention relates to the preparation and utilization of pretreated iron oxide magnetic pigments which have improved stability against mechanical attack such as attrition. This invention is particularly directed to improving the dispersibility of iron oxide magnetic pigments so that they can be readily dispersed within magnetic coatings to provide coatings having improved magnetic and sound properties.
2. DISCUSSION OF THE PRIOR ART
It is known that iron oxide pigments can be hydrophobized by means of alkyl halogen silanes at a temperature above 200.degree. C. in the presence of water vapor, approximately the following reaction taking place: EQU R--SiCl.sub.3 +H.sub.2 O.fwdarw.RSi(OH).sub.3 +3 HCl.
This process, however, is not satisfactory because, among other reasons, large amounts of hydrochloric acid have to be disposed of in such preliminary treatment with alkyl trichlorosilanes (German "Offenlegungsschrift" No. 1,767,973). In the same "Offenlegungsschrift" it is stated that hydrophobation out of aqueous solution has not been successful, or has succeeded to an unsatisfactory degree.
Pretreatment of iron oxide with polysiloxanes or with dialkyl or dialkylene silanes does not give satisfactory results, either (Japanese Patent Publication No. 7 4014-749).
It is an object of this invention, therefore, to provide a process for disposing a hydrophobic coating on an iron oxide pigment. It is a further object of this invention to provide a silica-containing coating on an iron oxide magnetic pigment to thereby improve the pigment's resistivity against mechanical attack such as attrition. It is a further object of this invention to improve the dispersibility of such pigments in an organic medium such as those employed in magnetic coatings.